1. Field of the Invention
The present invention relates to an engine control device, and more particularly to an engine control device which is used when hydraulic equipment such as a hydraulic pump is driven by an engine.
2. Description of the Related Art
Construction machines such as hydraulic excavators, bulldozers, dump trucks and wheel loaders have a diesel engine mounted thereon.
The structure of a conventional construction machine 1 will be described briefly with reference to FIG. 1. As shown in FIG. 1, a hydraulic pump 6 is driven by a diesel engine 2 used as a drive source. For the hydraulic pump 6, a variable displacement hydraulic pump is used, and its displacement D (cc/rev) is varied by varying the inclination angle of its swash plate 6a, or the like. Pressure oil discharged with a discharge pressure P and a flow rate Q (cc/min) from the hydraulic pump 6 is supplied to individual hydraulic actuators 31 to 35, e.g., a boom hydraulic cylinder 31, via operation valves 21 to 25. The supply of the pressure oil to the hydraulic actuators 31 to 35 drives these hydraulic actuators 31 to 35, and a working machine comprising a boom, an arm and a bucket and a lower traveling body which are connected to the hydraulic actuators 31 to 35 operate accordingly.
While the construction machine 1 is operating, a load applied to the working machine and the lower traveling body is constantly variable depending on the quality of excavated soil, the inclination of a running road or the like. Accordingly, the load of the hydraulic equipment (hydraulic pump 6) (hereinafter called as the hydraulic equipment load), namely the load applied to the engine 2, is changed.
The power (horsepower; kw) of the diesel engine 2 is controlled by adjusting an amount of fuel injected into the cylinders. This adjustment is made by controlling a governor 3 attached to a fuel injection pump of the engine 2. For the governor 3, an all-speed governor is generally used and adjusts an engine speed N and a fuel injection amount (torque T) according to the load to keep a target engine speed set by a fuel dial. In other words, the governor 3 adjusts the fuel injection amount to eliminate a difference between the target engine speed and the engine speed.
FIG. 14 shows a torque diagram of the engine 2, in which the horizontal axis represents an engine speed N (rpm; rev/min), and the vertical axis represents torque T (N·m).
The area specified by a maximum torque line R2 in FIG. 14 indicates the performance delivered by the engine 2. The governor 3 controls the engine 2 in such a manner that the torque T does not exceed the maximum torque line R2 to have an exhaust smoke limit or the engine speed N does not exceed a high idle rotational speed NH to have an over speed. The power (horsepower) of the engine 2 becomes maximum at a rated point V2 on the maximum torque line R2. “J” indicates an equal horsepower curve showing that the horsepower absorbed by the hydraulic pump 6 becomes equal horsepower.
When the maximum target rotational speed is set by the fuel dial, the governor 3 adjusts the speed on a maximum speed regulation line Fe which connects the rated point V2 and the high idle point NH.
A matching point, at which the power of the engine 2 and pump absorption horsepower are balanced as hydraulic equipment load increases, moves toward the rated point V2 along the maximum speed regulation line Fe. When the matching point moves toward the rated point V2, the engine speed N is decreased gradually, and the engine speed N becomes a rated rotational speed NR at the rated point V2.
Regulation lines Fe-1, Fe-2 . . . are sequentially determined as the target rotational speed set by the fuel dial becomes small, and the speed is adjusted on the individual regulation lines.
The engine 2 of the construction machine 1 is required to have adequate responsivity when the hydraulic equipment load becomes high. In other words, the engine has higher responsivity as a time, in which the matching point moves from the no load high idle point NH to the maximum load rated point V2 along the regulation line Fe, is shorter.
In this connection, a conventional engine control method gradually decreases the engine speed N when the matching point moves toward high load side along the regulation line Fe as described above. When the engine speed N lowers, the power accumulated in the flywheel of the engine 2 is momentarily released to the outside, and the appearance power of the engine 2 becomes larger than the real power. Therefore, the conventional engine control method is said to have good responsivity.
As described above, according to the conventional engine control method, the engine 2 can be made to follow the hydraulic equipment load with good responsivity but has disadvantages that a fuel consumption rate is large (bad) and pump efficiency is low. The fuel consumption rate (hereinafter called as the fuel consumption) is an amount of fuel consumption for power of 1 kW in one hour and an index for the efficiency of the engine 2. And, the pump efficiency is the efficiency of the hydraulic pump 6 which is specified by capacity efficiency and torque efficiency.
In FIG. 14, “M” indicates an equal fuel consumption curve. The fuel consumption becomes minimum at M1 forming a valley of the equal fuel consumption line M and becomes large from the fuel consumption minimum point M1 toward the outside.
It is apparent from FIG. 14 that the regulation line Fe is equivalent to an area where the fuel consumption is relatively large on the equal fuel consumption curve M. Therefore, the conventional engine control method had a disadvantage that the fuel consumption was large (bad) and not desirable in view of the engine efficiency.
Meanwhile, the variable displacement hydraulic pump 6 is generally known that, when the discharge pressure P is same, the capacity efficiency and the torque efficiency become high as the pump displacement D (swash plate inclination angle) is large, and the pump efficiency is high.
It is also apparent from the expression (1) below that, when the flow rate Q of the pressure oil discharged from the hydraulic pump 6 is same, the pump displacement D can be increased when the rotational speed N of the engine 2 is lowered. Therefore, the pump efficiency can be increased when the engine 2 is lowered its speed.Q=N·D  (1)
Therefore, the engine 2 is advisably operated in a low-speed area, where the rotational speed N is low, to enhance the pump efficiency of the hydraulic pump 6.
It is apparent from FIG. 14, however, that the regulation line Fe is equivalent to a high-speed area of the engine 2. Therefore, the conventional engine control method has a disadvantage that the pump efficiency is low.